1) Field of the Invention
This invention relates to a moving-magnet type linear d.c. brushless motor, which is one linear d.c. brushless motor in itself, but has plural moving elements.
2) Description of the Related Art
In small-size and high-precision linear d.c. brushless motors, there are moving-magnet type linear d.c. brushless motors having one moving element provided with a field magnet, which has P (P being an integer of 1 or higher) contiguous magnetic poles of alternating N and S polarity, and a stator armature arranged on a position of the stationary side facing the field magnet of the moving element through an air gap and a position sensor, which is a magnetoelectric transducer as a pole-discriminating sensor for the field magnet and is disposed on the stator armature.
As a result of a recent technical demand, not only one moving element but plural moving elements have however been required in some instances.
However, this is not so simple as a plurality of moving elements can be caused to travel separately and independently under control if the number of moving elements is only increased to a plurality in order to satisfy such a demand.
Namely, it is necessary to servocontrol the plural moving elements separately and independently with respect to different-speed moving, positioning and the like so as to drive them. However, since a position sensor provided on the stator armature detects the predetermined magnetic pole of field magnets in the plural moving elements, the plural moving elements will simultaneously travel arbitrarily if electric currents should be fed to their corresponding armature coils at the same time, resulting in troubles such as their collision.
When an attempt to make use of electric circuits is made in order to avoid such troubles, systems for a positioning control circuit, software of a microcomputer, etc. for such a purpose become very complex, so that the final cost becomes greatly higher and besides, such systems are unsuitable for practical use.
In addition, since the plural moving elements are required to be driven separately and independently under control, in some cases, they may be caused to travel in separate and independent traveling modes different from one another or alternatively, some of them may be caused to travel and the remainder may be caused to stop. In such cases, it is necessary to consider a sufficient counterplan for the above troubles.
Here, in order to separately detect the position and speed of the plural moving elements, it is necessary to provide a plurality of displacement detecting means such as encoders for detecting the movement of the moving elements corresponding thereto.
Incidentally, if the above-described encoders, power circuits and microcomputer are used upon driving the individual moving elements, the plural moving elements can be driven separately and independently under servocontrol by feeding electric currents to a group of armature coils of the stator armature, which face the field magnets of the plural moving elements, in predetermined directions even if no group of position sensors are present.
The linear d.c. brushless motor of the type making no use of any position sensors as described above is useful because groups of position sensors such as many expensive Hall elements can be omitted and it can hence be fabricated economically.
The linear d.c. brushless servomotor of the type making no use of any position sensors has the abovedescribed merits. On the other hand, it is however accompanied by defects due to the absence of position sensors groups compared with the linear d.c. brushless motor making use of the position sensors.
Namely, compared with the linear d.c. brushless motor making use of the position sensors, the linear d.c. brushless servomotor of the type making no use of the position sensors is little in reliability from a viewpoint of starting characteristics due to the absence of the position sensors. In addition, it is also accompanied by the same defect as described above from the influence of load characteristics. Accordingly, such a brushless servomotor is yet in the stage where a further improvement must be brought about.
Moreover, in the case where a plurality of moving elements are present, the linear d.c. brushless servomotor of the type making no use of the position sensors becomes very complex in electric circuit or software compared to the case of single moving element. It is hence accompanied by a defect that difficulties are encountered on the achievement of the desired purpose.
This is due to the fact that the linear d.c. brushless servomotor of the type making no use of the position sensors is not yet thoroughly studied and developed at this point of time.
Therefore, in order to control separately and independently the plural moving elements, the present inventor has used a system making use of position sensors in a moving-magnet type linear d.c. brushless motor of a polypolar polyphase system making use of plural moving elements, which have each a polypolar field magnet, and of numerous armature coils even if such a system is somewhat expensive in mechanical construction only for the cost of the position sensors.
In the case of the linear d.c. brushless motor making use of the position sensors as described above and having the plural moving elements, if the motor should be of the conventional type, the plural moving elements will move arbitrarily, resulting in a cause of troubles. Accordingly, the present inventor attempted first of all to use two moving elements and to provide with a first group of position sensors for detecting only a magnetic pole of a field magnet in a first moving element and a group of second position sensors for detecting only a magnetic pole of a field magnet in a second moving element so as to avoid the simultaneous movement of the first and second moving elements.
In one example thereof, it was possible to avoid the simultaneous movement by switching the first group of the position sensors and the second group of the position sensors from each other by a mechanical change-over switch in accordance with signals from a controller. However, such a method was accompanied by a defect that the mechanism of the change-over switch becomes complex and the cost of the motor hence comes high. In addition, when the number of the moving elements is at most 2, this method becomes possible because two groups of the position sensors can be arranged respectively on both sides of a stator armature. However, when the number of the moving elements becomes three or more, such a motor is accompanied by improper operation because some groups of the position sensors, which are used respectively for plural moving elements, must be arranged on the same side. It was hence difficult to construct a linear d.c. brushless motor having plural moving elements as a matter of fact.